Decomposition mechanism of Al1−xSixNy solid solution and possible mechanism of the formation of covalent nanocrystalline AlN/Si3N4 nanocomposites

Using a combined ab initio density functional theory (DFT) and thermodynamic modeling, we study the stability of a variety of phases and the possible mechanism of the decomposition of the Al1−xSixNy solid solution and formation of nanocrystalline AlN/Si3N4 nanocomposites, which have been experimentally investigated in a number of recent publications. It is shown that the linear and exponential dependence of the interaction parameter on temperature yields reliable results. The hexagonal close-packed (hcp)(ZnS) to hcp(β) phase transition points occur at x ∼ 0.36. The calculated temperature–composition diagrams show that spinodal decomposition mechanism is unlikely in this system because of too small de-mixing energy, which is comparable with the interfacial energy of semi-coherent interfaces. Thus, the decomposition should occur by nucleation and growth, accompanied by a phase transformation from the unstable hcp(ZnS)-SiN to stable hcp(β) or amorphous Si3N4, which probably limits the achievable hardness enhancement of the nanocomposites as compared with the nanocrystalline TiN/a-Si3N4 ones (where a indicates X-ray amorphous, and the stoichiometry Si3N4 symbolizes the fact that Si is fourfold coordinated to nitrogen, as in stoichiometric silicon nitride)